It is well known in the art that the performance of metal oxide semiconductor (MOS) devices may be enhanced by forming each individual MOS field effect transistor (FET) in a semiconductor island such as silicon on an insulator substrate so that each MOSFET is isolated from other MOSFET's.
Various substrate materials including sapphire are well known in the art, as set forth in the following patents and publications, the disclosures of which are expressly incorporated herein by reference: U.S. Pat. No. 3,393,088 to Manasevit, et al., U.S. Pat. No. 3,414,434 to Manasevit, U.S. Pat. No. 3,475,209 to Manasevit, U.S. Pat. No. 3,515,576 to Manasevit, U.S. Pat. No. 3,664,866 to Manasevit, U.S. Pat. No. 3,508,962 to Manasevit et al., M. W. Geis, et al., "Crystallographic Orientation of Silicon on an Amorphous Substrate Using an Artificial Surface Relief Grating and Laser Crystallization", Applied Physics Letters, Volume 35 No. 1, pgs. 71-74, (July 1, 1979, T. E. Kamins et al., "A Monolithic Integrated Circuit Fabricated in Laser Annealed Polysilicon", I.E.E.E. Transactions on Electron Devices, Volume Ed. 27, No. 1, (January, 1980), pgs. 290-295.
Fabrication of such silicon on sapphire (SOS) devices is accomplished by growing epitaxial silicon on a sapphire substrate and then etching the silicon to form individual islands on which individual MOSFETS may be fabricated. A significant problem in the art of silicon on sapphire (SOS) is the low channel mobility which results from a high defect density at the top epitaxial silicon surface. Reduction in channel mobility decreases the device operating speed.
Another problem is that aluminum conductors interconnecting between individual MOSFETS on the sapphire substrate must cover a nearly vertical step formed at the edge of each silicon island. A reduction in the thickness of the deposited aluminum conductor and cracks in the aluminum may occur in the vicinity of a vertical step. Such cracks and reduction in the thickness of the deposited aluminum increases the likelihood of discontinuties in the aluminum conductor which cause device failure.
Another problem which is peculiar to SOS devices is that growth of a silicon dioxide layer over the silicon island usually is accompanied by a region of reduced oxide thickness resulting in a "V"-shaped groove at the lower corner of each silicon island edge. If a polycrystalline silicon conductive layer is deposited or grown thereover, the amount of insulation between the overlying polysilicon conductor and the silicon island is greatly reduced at the V-shaped groove, thereby increasing the likelihood of breakdown conduction through the insulating oxide film between the polysilicon conductor and the silicon island.